In general, an ignition plug includes an insulator having an axial hole extending in an axial line, a center electrode inserted in a forward end portion of the axial hole, a metallic shell provided around the insulator, and a ground electrode providing at a forward end portion of the metallic shell and forming a spark discharge gap in cooperation with the center electrode. When a predetermined voltage is applied to the spark discharge gap, spark discharge occurs at the spark discharge gap, whereby an air-fuel mixture or the like is ignited.
The insulator has a leg portion which is formed at the forward end and has a relatively small diameter, and a tapered engagement portion provided adjacent to the rear end of the leg portion. The metallic shell has, on its outer circumference, a screw portion used for attaching the ignition plug to an internal combustion engine or the like. A flange-shaped seating portion is formed on the rear end side of the screw portion, and a cylindrical tube portion (screw neck) is formed between the screw portion and the seating portion. In addition, the metallic shell has a protrusion which protrudes from its inner circumferential surface toward the radially inner side. The metallic shell and the insulator are fixed together in a state in which the engagement portion of the insulator is engaged with the protrusion of the metallic shell directly or indirectly via a sheet packing or the like (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2005-183177). Notably, the heat exerted on forward end portions of the leg portion and the center electrode as a result of combustion of an air-fuel mixture or the like is mainly conducted to the engagement portion through the leg portion and the center electrode, and is conducted from the engagement portion to the protrusion.
In recent years, a reduction in the diameter of an ignition plug (metallic shell) has been demanded in order to increase the degree of freedom of the engine layout or for other reasons. In such an ignition plug having a reduced diameter, the outer diameters (volumes) of the insulator and the center electrode disposed inside the metallic shell must be decreased. Therefore, the heat conduction path becomes narrow, and the heat conduction performance may deteriorate. If the heat conduction performance deteriorates, the leg portion and the center electrode are overheated, which may lead to, for example, a decrease in the yield strength of the insulator (leg portion), generation of pre-ignition in which a forward end portion of the insulator (leg portion) serves as a heat source, and rapid erosion and deformation of the center electrode. One possible measure for preventing the overheating of the insulator and the center electrode is decreasing the thickness of the metallic shell so as to increase the inner diameter of the metallic shell to thereby increase the outer diameters (volumes) of the insulator, etc.
However, in the case where the thickness of the metallic shell is merely reduced, when a tightening torque is applied to the metallic shell in order to screw the screw portion into an internal combustion engine or the like, the metallic shell may break at a tube portion thereof.
The present invention has been accomplished in view of the above-described problem, and its advantage is an ignition plug which can effectively enhance the heat conduction performances of an insulator and a center electrode to thereby suppress overheating of the insulator, etc., while preventing breakage of a tube portion of a metallic shell more reliably.